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13 January 2013

BOBGRAM issued 13 Jan 2012

WEATHERGRAM
YOTREPS
Issued 13 Jan 2013
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) was hugging the plus 0.5 value in Sep Oct Nov, and during the last few weeks it dived to almost minus 1, but as at 13 Jan has relaxed again to near zero.

The Ocean: Sea surface temperatures across the equatorial Pacific act as a thermostat for the planetary weather engine. When they are warmer than normal ( El Nino) we get more evaporation and clouds in that region and this augments the earthly wind zones and has an impact on the latitude zones of weather.

Well, in the past month the SST in this target zone has mostly been slightly below normal, and the oceanic pattern is neither El Nino, nor La Nina; it remains neutral.

The latest SST anomaly data also shows there is a much warmer than around the seas neighbouring NW Australia. This is the source area for TC NARELLE which is side-swiping west Australia this week.

WEATHER ZONES
The monsoonal trough is sitting over central Australia as is normal for this time of year, but its clouds and rain are late—may arrive later this month—and so Australian continent has become something like a hot plate, heating up in the sun. This explains its continuing heat wave.

The latest pressure and rain map for the region shows this, along with path so far of TC NARELLE and some likely future paths of that system. It also shows the STR Sub tropical ridge from Aussie bight to north of NZ, and the SPCZ South Pacific convergence zone, which is now covering a wide latitude band.

The tropical Low near Tonga is expected to deepen over next few days over NIUE and SOUTHERN COOKS and then go off to the south. BRACE FOR IT.

TASMAN SEA/NZ TROUGHS
The incoming jetstreams has been crossing the mid Tasman Sea. This has been controlling the fronts crossing NZ--- they have been very active over the South Island, but weakening over the North Island.